In plain English
The context window is how much the assistant can hold in mind at once, in one conversation. Your messages, its replies, any files you’ve pasted in, that’s all sitting inside it. There’s a limit, and once you hit it, the oldest stuff starts falling off the edge.
That’s the bit people notice as “forgetting”. You mentioned something important ten messages ago, the chat’s moved on, and suddenly it’s acting like you never said it. It hasn’t ignored you on purpose, it just literally doesn’t have that part in view any more. A fresh chat, meanwhile, starts completely empty, no memory of anything from before.
A worked example
Picture a long back-and-forth editing a document, forty messages deep. You ask it to check something against a rule you set at message three, and it can’t, because message three has scrolled out of the window entirely. Frustrating, but not a fault, just the limit doing what limits do.
The fix that actually works is summarising in chunks: paste a section, get feedback, summarise what was agreed, carry that summary forward instead of the whole history. Big documents especially might not fit into the window at all in one go, so breaking them up isn’t a workaround, it’s basically required.
The usual mix-up
People assume the assistant remembers everything, across every chat they’ve ever had with it. Unless a memory feature has specifically been switched on, that’s not true, each new chat starts from zero. And even inside a single long chat, the window still has a hard edge somewhere. It’s not infinite patience. It’s a room with walls.